U.S. patent number 6,421,202 [Application Number 09/571,921] was granted by the patent office on 2002-07-16 for hard disk drive magnetic head coated with low surface energy material.
This patent grant is currently assigned to International Business Machines Corporation. Invention is credited to Masayuki Kanamaru, Masahiro Nagayoshi, Hidetsugu Tanaka.
United States Patent |
6,421,202 |
Tanaka , et al. |
July 16, 2002 |
Hard disk drive magnetic head coated with low surface energy
material
Abstract
At least part of the surface of a read/write element of the
magnetic head surface is coated with low surface energy materials.
The low surface energy materials have sufficient humidity
resistance, wetting resistance, and hydrophobicity to repel water
from the coated surfaces. Such properties yield improved corrosion
resistance for the coated surfaces. The low surface energy
materials include fluorocarbon compounds (FCOC) and special
materials such as perfluoroalkylpolyoxyethyleneethanol. The low
surface energy materials are coated on the magnetic heads as thin
films. The thin films have a thickness in the range of 5-100
angstroms, but are preferred in the range of 10-20 angstroms. The
thin films provide sufficient adhesion on and corrosion protection
of the heads without decreasing their performance.
Inventors: |
Tanaka; Hidetsugu (Fujisawa,
JP), Kanamaru; Masayuki (Fujisawa, JP),
Nagayoshi; Masahiro (Fujisawa, JP) |
Assignee: |
International Business Machines
Corporation (Armonk, NY)
|
Family
ID: |
15720830 |
Appl.
No.: |
09/571,921 |
Filed: |
May 16, 2000 |
Foreign Application Priority Data
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Jun 8, 1999 [JP] |
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11-160711 |
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Current U.S.
Class: |
360/122;
360/235.2; G9B/5.143; G9B/5.034 |
Current CPC
Class: |
G11B
5/10 (20130101); G11B 5/40 (20130101); G11B
5/60 (20130101); G11B 11/1058 (20130101) |
Current International
Class: |
G11B
5/10 (20060101); G11B 5/40 (20060101); G11B
11/105 (20060101); G11B 5/60 (20060101); G11B
11/00 (20060101); G11B 005/40 () |
Field of
Search: |
;360/122,235.1,235.2,235.3 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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59162613 |
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Sep 1984 |
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JP |
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61087209 |
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May 1986 |
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JP |
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6-259911 |
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Sep 1994 |
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JP |
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WO 94/08334 |
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Apr 1994 |
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WO |
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WO 97/22119 |
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Jun 1997 |
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WO |
|
Primary Examiner: Ometz; David L.
Attorney, Agent or Firm: Knight; G. Marlin Bracewell &
Patterson, L.L.P.
Claims
What is claimed is:
1. A magnetic head having at least a part of a surface thereof
coated with materials having a low surface energy; and wherein the
materials are fluorocarbon compounds containing
perfluoroalkylpolyoxyethyleneethanol.
2. The magnetic head of claim 1 wherein the surface energy of said
materials with low surface energy is in the range of 12-15
erg/cm.sup.2.
3. The magnetic head of claim 1 wherein the coating of said
materials with low surface energy is in the range of 5-100
angstroms.
4. The magnetic head of claim 1 wherein the coating of said
materials with low surface energy is in the range of 10-20
angstroms.
5. The magnetic head of claim 1 wherein the magnetic head is a
write element.
6. The magnetic head of claim 1 wherein the magnetic head is a read
element.
7. A carriage assembly having a magnetic head selected from a group
consisting of a read element and a write element, wherein the
magnetic head has at least a part of a surface thereof coated with
materials having a low surface energy; and wherein the materials
are fluorocarbon compounds containing
perfluoroalkylpolyoxyethyleneethanol.
8. The carriage assembly of claim 7 wherein the surface energy of
said materials with low surface energy is in the range of 12-15
erg/cm.sup.2.
9. The carriage assembly of claim 7 wherein the coating of said
materials with low surface energy is in the range of 5-100
angstroms.
10. The carriage assembly of claim 7 wherein the coating of said
materials with low surface energy is in the range of 10-20
angstroms.
11. A hard disk drive having a carriage assembly with a magnetic
head selected from a group consisting of a read element and a write
element, wherein the magnetic head has at least a part of a surface
thereof coated with materials having a low surface energy; and
wherein the materials are fluorocarbon compounds containing
perfluoroalkylpolyoxyethyleneethanol.
12. The hard disk drive of claim 11 wherein the surface energy of
said materials with low surface energy is in the range of 12-15
erg/cm.sup.2.
13. The hard disk drive-in of claim 11 wherein the coating of said
materials with low surface energy is in the range of 5-100
angstroms.
14. The hard disk drive of claim 11 wherein the coating of said
materials with low surface energy is in the range of 10-20
angstroms.
Description
BACKGROUND OF THE INVENTION
1. Technical Field
The present invention generally relates to disk recording devices,
and more particularly to technology for protecting magnetic heads
from corrosion by applying a coating to magnetic heads used in hard
disk drives.
2. Description of the Related Art
In recent years, the recording capacity of disk recording devices
has increased very rapidly. This is significant in typical magnetic
recording hard disk drives (HDD). As the size of HDDs has decreased
their storage density has increased. The high performance/high
sensitivity write (recording) and read elements used in HDDs are
susceptible to environmental influences. In particular, corrosion
is a critical issue in maintaining HDD reliability.
One of the main causes of corrosion is believed to be water that is
present within the HDDs. Although the water can be removed by
placing a drying agent inside the HDD, the absorbency of drying
agents is limited. Thus, drying agents have the drawback of being
unable to serve for long periods of time.
Another prior art solution suggested coating the surfaces of the
magnetic disks with a lubricant. The lubricant is transferred and
supplied to the read/write elements and substantially covers areas
susceptible to corrosion. However, this approach also had the
drawback of being unable to reliably serve for long periods of
time, as it is difficult to ensure retention of the lubricants for
extended periods.
Another solution uses protective coating layers on the read/write
elements to improve abrasion resistance. However, such coatings are
so thick that they reduce the recording properties of the
read/write elements. It is not yet possible to form a thin,
protective film coating on the elements without diminishing their
recording properties.
Yet another solution uses highly corrosion-resistant materials to
form the read/write elements themselves. Unfortunately, this
solution makes it difficult to ensure sufficient corrosion
resistance while maintaining adequate function for reading and
writing to the disks.
SUMMARY OF THE INVENTION
At least part of the surface of a read/write element of the
magnetic head surface is coated with low surface energy materials.
The low surface energy materials have sufficient humidity
resistance, wetting resistance, and hydrophobicity to repel water
from the coated surfaces. Such properties yield improved corrosion
resistance for the coated surfaces. The low surface energy
materials include fluorocarbon compounds (FCOC) and special
materials such as perfluoroalkylpolyoxyethyleneethanol. The low
surface energy materials are coated on the magnetic heads as thin
films. The thin films have a thickness in the range of 5-100
angstroms, but are preferred in the range of 10-20 angstroms. The
thin films provide sufficient adhesion on and corrosion protection
of the heads without decreasing their performance.
It is the main purpose of the present invention to ensure
reliability of HDDs by improving corrosion resistance of magnetic
heads in HDDs.
It is another purpose of the present invention to select suitable
materials for highly corrosion resistant coating.
It is a further purpose of the present invention to form thin films
with correct thickness as the coating for magnetic heads.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is an exploded perspective view of a hard disk drive
constructed in accordance with the invention.
FIG. 2 is a perspective view of a slider of the hard disk drive of
FIG. 1 while "flying" above a disk.
FIGS. 3A-3C are schematic views of three wetting states on a
reference surface for illustrating surface energy.
FIG. 4 is a plot showing output data obtained from magnetic heads
coated in accordance with the invention while left standing under a
high temperature and high humidity environment for periods A and
B.
FIG. 5 is a plot showing output data obtained from prior art,
uncoated magnetic heads used in a conventional technology at an
initial stage before exposure to a high temperature and high
humidity environment, and then after left standing under a high
temperature and high humidity environment.
DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENT
FIG. 1 is an exploded perspective view of a hard disk drive (HDD)
10. In HDD 10, a plurality of disks 12 are rotated as a whole by a
spindle motor 14 during operation. A carriage assembly 16 is
included as an element configuring HDDs, and the carriage assembly
16 crosses over disks to access disks 12. This movement of the
carriage allows data to be written into and read from the desired
parts of the disks 12.
There are small holes 20 in the top cover 18, and a breather filter
22 is placed to cover the small holes 20, which can remove dust
from outside the HDDs. Therefore, air can go into and out of HDD
through these small holes 20 and the breather filter 22. For
example, a HEPA filter is used as the breather filter 22. Thus,
environment outside HDD is transferred inside.
As shown in FIG. 2, a slider 24 is attached at the end of the
carriage assembly 16 (FIG. 1). For example, AlTiC is used as a
material for slider 24. Flow of the air F caused by rotation of
disk 12 flows upstream to downstream of the slider, forming air
bearing surface (ABS) 26 and keeping the slider 24 floating above
the disks 12. As the slider 24 is typically tapered or chamfered so
that its upstream side is higher than its downstream side, the
downstream side becomes closest to the disks 12.
A write/read element 30 is attached on downstream side of the
slider, and the data is written into or read from disks 12 through
electromagnetic effect M between this element 30 and the surface of
disks 12. For full function of the electromagnetic effect M, it is
preferable that the distance between the write/read element 30 and
the disks 12 is as short as possible. This is because
electromagnetic effect M can be localized at the surface of disks
12 to be utilized at high density. However, physical corrosion of
the write/read element 30 and the rotating disks 12 is not
allowed.
In FIG. 2, vicinity of the write/read element 30 is enlarged and
shown more precisely. As shown here the write/read element 30 is
often used separately as a write element 32 and a read element 34.
Furthermore, in some cases materials 36 other than the material
(AlTiC) for the slider 24 are located on a part of the surface of
the slider 24 as a material to serve as a foundation for the
write/read element 30. There may be a similar case where vicinity
of this element 30 is surrounded by materials 36 other than the
material for the slider 24, even though they do not serve as the
foundation for this.
In either case, the "surface" of the write/read element 30 is
exposed to ABS as shown in the figure, for full function of the
electromagnetic effect M. However, it has not necessarily to be
exposed directly to ABS and it may be enough that it is exposed to
the downstream side of the slider, for example even to the rear
side 38 (behind the scene in the figure, though can be seen from
the direction depicted by an eye and an arrow).
In the description of the present invention, a slider including
either write element 32 or read element 34 is called "magnetic
head" for convenience. Typically, high permeability alloy is used
for the write elements 32 and alloy utilizing magnetic resistance
(MR) effect is used for the read elements 34. Thus, either element
includes metal parts. As these metal parts are necessarily exposed
to surfaces of ABSs, they are susceptible to corrosion by the air
flow F, etc., and once corroded, the electromagnetic properties are
significantly deteriorated. Especially reduction in output of
reading signals from the read element 34 is significant. Also, the
write element 32 cannot give the disks enough electromagnetic
effect, resulting in poor write in some cases.
When humidity increases inside the HDDs, surfaces of these
write/read elements 30 become more apt to be exposed to humidity.
This causes significant corrosion, as described above. Small
amounts of corrosive gases generated within HDDs are believed to be
related to other causes of corrosion. Generation of such gases may
be related to volatile components generated from parts themselves
in the HDD, or related to aging of the HDD parts. Actually
corrosion is believed to proceed due to electrochemical processes
related to variety of those causes. In addition, high temperature
inside HDDs will further enhance corrosion reactions. High
temperature will also strongly cause generation of corrosive
gases.
A surface energy index is an indication of resistance of a given
surface against water. According to this index, the degree of
humidity resistance or wetting resistance, (Anti Wetting) a degree
of rejecting water, may be known from the condition of water on a
reference surface.
FIGS. 3A, 3B, and 3C are schematic views of three categorized
states of water on a surface. The parts of the material that are
dotted represent states of water, and the parts of the material
that are shaded represent the reference surfaces. The states of
water are "complete" wetting for FIG. 3A, "good" wetting for FIG.
3B, and "poor" wetting for FIG. 3C. These expressions are only for
convenience. Those skilled in the art may as well use other
expressions. From the point of view that it is good to be wet, FIG.
3A is preferable and it is unpreferable to be closer to FIG. 3C. On
the contrary from the point of view that degree of wetting
resistance is the issue as in the present invention, FIG. 3C is
preferable and it is not preferable to be closer to FIG. 3A. This
may be apparent form the meaning of terms "hydrophobicity and water
repellence."
The states of FIGS. 3A, 3B, and 3C are categorized by contact
angles .theta.. Contact angle .theta. is defined as the angle
.theta. formed between the tangent line and the reference surface
at the contact point of water in the form of a drop of water and
the reference surface. The contact angle .theta. is determined by
the following equation using surface energy of water .gamma..sub.t
and the surface energy of reference surface .gamma..sub.s. .THETA.
in the equation is an experimental value.
Contact angle .theta. can be determined experimentally by taking
the image of a drop of water in a CCD camera and processing the
image. This equation shows that it is preferable from the point of
view of humidity resistance or wetting resistance that the surface
energy of the reference surface .gamma..sub.s is small enough as
compared with the surface energy of water .gamma..sub.t as far as
the latter is constant. The surface energy of water is
.gamma..sub.t =72.2 erg/cm.sup.2. On the other hand when the
material of the reference surface is alumina (Al.sub.2 O.sub.3 :
aluminum oxide), the surface energy of alumina is .gamma..sub.s
=169 erg/cm.sup.2.
The reason why the surface energy of alumina is the issue will be
mentioned here. In other words, it is the reason why the write/read
element 30 which must be protected from corrosion is not the direct
issue as the reference surface. Although vicinity of the write/read
element 30 is surrounded by other materials 36 in some cases as
described above, the reference surface here corresponds to the
other material 36 which is comprised of alumina (FIG. 2).
Thus, in order to evaluate humidity resistance, it is more
reasonable to focus on the surface energy of alumina that occupies
most of the surface area. Exposed areas of the write element 32 and
read element 34 will total only to- a very small exposed area. It
is noted that the exposed areas of the write element 32 and read
element 34 in FIG. 2 are exaggerated to look bigger for convenience
of illustration. In other words, it is reasonable to evaluate the
contact angle .theta. at the contact points of water and
alumina.
In the present invention, coating was formed as a substitute for
substrate surface to reduce surface energy of the surface of the
substrate, and the coating was provided with humidity resistance
and wetting resistance. FCOCs (fluorocarbon compounds) were
selected as the suitable material for the thin film. A material
containing perfluoroalkylpolyoxyethyleneethanol was selected as an
example of FCOCs (fluorocarbon compounds). The surface energy of
the thin film material was found to be .gamma..sub.s =12-15
erg/cm.sup.2. However, even lower surface energy is expected to be
realized by adding more perfluoroalkylpolyoxyethyleneethanol. Lower
limit even closer to zero can be experimentally estimated to exist.
Those skilled in the art may easily achieve such modification.
Thus, the selected materials may be utilized as-optimum anti
wetting agents. As the surface energy of alumina (Al.sub.2 O.sub.3
: aluminum oxide) is .gamma..sub.s =169 erg/cm.sup.2 as described
above, it is understood that substitution with far lower surface
energy reference surface is attained by this coating.
Another advantage of the FCOCs (fluorocarbon compounds) is that
they can form thin films as solid. The thin film as solid has been
assured experimentally to retain adhered to surfaces for a long
time. As actual formation of thin films, processes using
dipping-and-drawing and plasma CVD were carried out, and FCOC was
confirmed to be effective in both processes. Description of these
processes is omitted since those skilled in the art may easily add
and reproduce them. Also, those skilled in the art may easily adopt
other formation processes such as sputtering, vapor deposition, and
coating. Also the process may be developed to realize smoother
surface by changing coarseness or the feature of the surface using
physical surface treatment methods such as ion beam etching.
In the present invention it was found that coating of
perfluoroalkylpolyoxyethyleneethanol is preferably formed into thin
films with the thickness of 5-100 angstroms. Optimum range of
thickness is determined by following reasons. If the thin film is
too thin, enough effect of corrosion resistance may not be
expected. Also some parts may be left uncovered by the thin film
due to technological limitation of thin film formation. Experiments
showed that thickness of at least 5 angstroms is necessary.
On the other hand, if the thin film is too thick, function of the
magnetic head is damaged during operations of the HDDs. Thus the
extra thickness will substantially reduce the allowance in the
distance between disks 12 and the write/read elements 30. This
situation is undesirable since it is important to secure
electromagnetic effect M (FIG. 2) by reducing the distance between
them as described above. Therefore, a limitation in thickness is
also necessary. Experiments showed that thickness of 100 angstroms
is realized in RDDs presently. Needless to say that strong adhesion
for a very long period may be attained with such thickness. A range
of 10-20 angstroms was found to be more preferable by narrowing the
optimum thickness range, taking in consideration the balance
between such limitations of thinness and thickness.
As the range for thin film formation, it is enough to cover all the
surface in which the record/read elements 30 are exposed. That is,
the range will be sufficient where the purpose of the present
invention, namely, hydrophobicity and water repellence are
expected. On the other hand, it is more difficult to try to form
thin films only on these surfaces, because these surfaces are
extremely small exposed areas and do not have simple features as
shown enlarged and more precisely in FIG. 2.
Thus, in the present invention, thin film is formed so that some
parts of the other materials 36 where alumina is used are also
coated. This is in order to expect enough hydrophobicity and water
repellence for the exposed surfaces of the write element 32 and the
read element 34. This shows that it is appropriate to take the
surface energy of alumina as an object of comparison in evaluating
humidity resistance. It is because the contact angle .theta. is to
be evaluated at the contact point with alumina. Of course it is not
necessary to form thin film on parts of the write element or the
read element, if the parts are already provided with enough
corrosion resistance.
As depicted in FIG. 1, an entire HDD was left in a
temperature-humidity chamber out of operation, in order to examine
adhesive effect of the thin film. As the air can go into and out of
the HDD as described above, the environment outside HDD is
transferred inside. Generally, temperature and humidity have
significant accelerability for promoting reactions. Especially
humidity above between 80% and 90%. RH (relative humidity) shows
extraordinary acceleration. Humidity was set to be 90% RH (relative
humidity), and temperature was set to be 50.degree. C. Humidity of
90% RH is a condition in which dew formation is easy, and in which
it is highly probable for water vapor or water to contact surfaces.
Both the humidity and temperature conditions are fairly severe. For
example, an accelerative examination under such a condition for a
week has the significance of assuring that the object may be used
under ambient environment for about 5 long years.
The data points in the graph of FIG. 4 show output value data
obtained from read elements 34 of the magnetic heads. Coating of
the present invention is formed on each magnetic head. The
horizontal axis represents the output value data obtained from each
magnetic head left standing for the period A of more than a week,
and the vertical axis represents the output value data obtained
from each magnetic head left standing for the period B which is
twice the period A. Thus, this graph correlates the data after
standing for the period A and the data after standing for the
period B. As an evaluation, output values above the lower limit are
estimated to assure the performance. All the dots are shown to be
above the lower limit both in the horizontal and vertical axes.
Thus, it was confirmed that the reliability of HDDs was secured by
using magnetic heads coated with the thin film of the present
invention.
The dots in the graph of FIG. 5 show the result of an examination
of magnetic heads used in conventional technologies under a high
humidity and high temperature condition similar to FIG. 4. However,
coating of the present invention was not formed here. And the
horizontal axis of this graph represents data of output value
obtained from magnetic heads at the initial stage before exposed to
high temperature and high humidity environment (different from FIG.
4). Further the vertical axis of this graph represents output data
from magnetic heads after standing for the period A (different from
FIG. 4 in which the vertical axis represents data of the period
B).
In case of magnetic heads of conventional technology shown in FIG.
5, 4% of the dots were below the lower limit already after standing
for the period A. The ratio 4% can be found from the number of dots
below the lower limit of the vertical axis. Thus, influence of
corrosion is considered to have already occurred after standing for
the period A. Although not shown as a graph, 20% of the dots were
below the lower limit according to the output data obtained from
magnetic heads left standing under a high temperature and high
humidity environment for further period A (namely total of period
B). After this accelerative examination, the surface was physically
examined to see whether the formed thin film coating still
remained. Namely the adhesive power of the coating formed was
confirmed. As a result, detachment to be functionally problematic
inside HDDs was not confirmed, showing enough adhesive power for a
long period.
Though the present invention has been explained in relation to
magnetic disk elements, those skilled in the arts may easily
achieve application of the present invention for other disk record
elements such as photomagnetic record elements or phase transition
type record elements, and to the heads used in them. Magnetic heads
coated by low surface energy materials according to the present
invention was found to be very excellent in corrosion resistance.
Reliability of HDDs is secured by this.
* * * * *